grist for/to one's mill, something employed to one's profit or advantage, especially something seemingly unpromising: Every delay was so much more grist for her mill.
Origin: before 1000; Middle English, Old English; akin to Old English grindan to grind
O.E. grist "action of grinding, grain to be ground," perhaps related to grindan "to grind" (see grind). Meaning "wheat which is to be ground" is c.1430; the figurative extension from this sense is from the same date.